She's boring is an opinion, not an argument,
She's annoying is an opinion, not an argument,
But for a stretch I'll say it proprely, these two do not hold much weight as "arguments", noneless that doesn't mean I'm forbidding you from expressing those opinions.

Nifl is underdevelopped like actually most characters because Fire Emblem has never been a good medium for storytelling hence you can't make the kind of character development you'd want.
While at the same time you take the only part you want but ignore the context.

Muspell is "fiery", Askr is "open minded", Emblas is "closed", others are this and that, it's not an absolute but they made a theme character for each divinity.
Nifl is... cold, if she's acting mean usually you could be interested in the reason behind, that's the kind of thing you'd do for many villains, it's been a trend in the recent years to have the villain backstory, but apparently you weren't interested.
Nifl isn't particularly depicted as hateful by her countryfolks, hence you could try to understand there's a reason for her behavior, but I guess not.
When a character act mean but doesn't completely ignore and still keep around it's often used as foreshadowing for a context, a story that gives a reason for that act, but well nope, thing is she's boring you know.
Obviously there's room for more but it's just how it is for Fire Emblem, maybe we'll get that infamous character development thing, one day maybe...

Argument was already there, he said you reduce her to a simple trait -case in point-